While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, DUV and VUV lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using an ArF excimer laser as the light source is thought requisite to the micropatterning technique capable of achieving a feature size of 0.13 μm or less.
In the photolithography using an ArF excimer laser (wavelength 193 nm) as the light source, a high sensitivity resist material capable of achieving a high resolution at a small dose of exposure is needed to prevent the degradation of precise and expensive optical system materials. Among several measures for providing high sensitivity resist material, the most common is to select each component which is highly transparent at the wavelength of 193 nm. For example, poly(meth)acrylic acid and derivatives thereof, norbornene-maleic anhydride alternating copolymers, polynorbornene and metathesis ring-opening polymers have been proposed as the base resin. This choice is effective to some extent in that the transparency of a resin alone is increased.
Studies have also been made on photoacid generators. In prior art chemically amplified resist compositions for lithography using KrF excimer laser, photoacid generators capable of generating alkyl- or aryl-sulfonic acid are used. However, the use of these photoacid generators in chemically amplified resist compositions for ArF lithography results in an insufficient acid strength to scissor acid labile groups on the resin, a failure of resolution or a low sensitivity, and is not suited for the microfabrication of microelectronic devices.
For the above reason, photoacid generators capable of generating perfluoroalkylsulfonic acid having a high acid strength are generally used in ArF chemically amplified resist compositions. These photoacid generators capable of generating perfluoroalkylsulfonic acid have already been developed for use in the KrF resist compositions. For instance, JP-A 2000-122296 and U.S. Pat. No. 6,048,672 (or JP-A 11-282168) describe photoacid generators capable of generating perfluorohexanesulfonic acid, perfluorooctanesulfonic acid, perfluoro-4-ethylcyclohexanesulfonic acid, and perfluorobutanesulfonic acid. JP-A 2002-214774, US Patent Application 2003-0113659 A1 (JP-A 2003-140332) and US Patent Application 2002-0197558 A1 describe novel photoacid generators capable of generating perfluoroalkyl ether sulfonic acids.
On the other hand, perfluorooctanesulfonic acid and homologues thereof (collectively referred to as PFOS) are considered problematic with respect to their stability (or non-degradability) due to C—F bonds, and biological concentration and accumulation due to hydrophobic and lipophilic natures. The US EPA adopted Significant New Use Rule, listing 13 PFOS-related chemical substances and further listing 75 chemical substances although their use in the photoresist field is excluded. See Federal Register/Vol. 67, No. 47, page 11008/Monday, Mar. 11, 2002, and Federal Register/Vol. 67, No. 236, page 72854/Monday, Dec. 9, 2002.
Facing the PFOS-related problems, manufacturers made efforts to develop partially fluorinated alkyl sulfonic acids having a reduced degree of fluorine substitution. For instance, JP-A 2004-531749 describes the development of α,α-difluoroalkylsulfonic acid salts from α,α-difluoroalkene and a sulfur compound and discloses a resist composition comprising a photoacid generator which generates such sulfonic acid upon irradiation, specifically di(4-tert-butylphenyl)iodonium 1,1-difluoro-1-sulfonate-2-(1-naphthyl)ethylene. JP-A 2004-2252 describes the development of α,α,β,β-tetrafluoroalkylsulfonic acid salts from α,α,β,β-tetrafluoro-α-iodoalkane and sulfur compound and discloses a photoacid generator capable of generating such a sulfonic acid and a resist composition comprising the same. JP-A 2004-307387 discloses 2-(bicyclo[2.2.1]hept-2-yl)-1,1-difluoroethylsulfonic acid salts and a method of preparing the same.
The substances disclosed in these patents have a reduced degree of fluorine substitution, but suffer from several problems. They are insufficient from the standpoint of decomposition because they do not have decomposable substituent groups such as ester structure. A certain limit is imposed on the molecular design for changing the size of alkylsulfonic acid. The starting materials containing fluorine are expensive.
The ArF lithography started partial use from the fabrication of 130-nm node devices and became the main lithography since 90-nm node devices. Although lithography using F2 laser (157 nm) was initially thought promising as the next lithography for 45-nm node devices, its development was retarded by several problems including the quality of CaF2 single crystal used in projection lens, a need for hard pellicle that requires design changes of the optical system, and low etch resistance of resists. A highlight was suddenly placed on ArF immersion lithography. See Journal of Photopolymer Science and Technology, Vol. 17, No. 4, p587 (2004).
The resolution of a projection lens through which a pattern image is projected onto a wafer increases as its numerical aperture (NA) increases. In the immersion lithography, the intervention of a liquid having a higher refractive index than air between the projection lens and the wafer allows the projection lens to be designed to a NA of 1.0 or higher, achieving a higher resolution. As the liquid, water having a refractive index of 1.4366 is used while alcohols such as ethylene glycol and glycerin are under investigation.
Regrettably, the immersion lithography gives rise to the problem that the resist pattern after development can collapse or deform into a T-top profile. Also, minute water droplets are left on the resist and wafer after the immersion exposure, which can often cause damages and defects to the resist pattern profile. There exists a need for a patterning process which can form a satisfactory resist pattern after development according to the immersion lithography.